Power up detectors are circuits used to determine when a power supply has reached a certain minimum voltage. When a power supply is turned on it does not instantaneously reach its steady state operating voltage level. Rather, for a finite start up period or charging phase, the output of the power supply rises monotonically towards its operating voltage as it charges the load capacitance coupled to its output.
During this start up period, the voltage level of the power supply may not be sufficiently high enough to properly operate digital logic. As such, digital logic may not correctly resolve operands and propagate erroneous results from one stage to the next. Power up detectors are used to disable digital logic during this start up period until the power supply has reached a threshold logic level voltage below which the digital logic cannot be guaranteed to operate properly. Once the threshold logic voltage level has been reached, the power up detector enables the digital logic to commence normal operation.